[GitHub] [druid] sthetland commented on a change in pull request #9122: Add SQL GROUPING SETS support.

[GitBox <gi...@apache.org>]

sthetland commented on a change in pull request #9122: Add SQL GROUPING SETS support.
URL: https://github.com/apache/druid/pull/9122#discussion_r384101696
 
 

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 File path: docs/querying/sql.md
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 @@ -86,6 +86,22 @@ trigger an aggregation query using one of Druid's [three native aggregation quer
 can refer to an expression or a select clause ordinal position (like `GROUP BY 2` to group by the second selected
 column).
 
+The GROUP BY clause can also refer to multiple grouping sets in three ways. The most flexible is GROUP BY GROUPING SETS,
+for example `GROUP BY GROUPING SETS ( (country, city), () )`. This example is equivalent to a `GROUP BY country, city`
+followed by `GROUP BY ()` (a grand total). With GROUPING SETS, the underlying data is only scanned one time, leading to
+better efficiency. Second, GROUP BY ROLLUP computes a grouping set for each level of the grouping expressions. For
+example `GROUP BY ROLLUP (country, city)` is equivalent to `GROUP BY GROUPING SETS ( (country, city), (country), () )`
+and will produce grouped rows for each country / city pair, along with subtotals for each country, along with a grand
+total. Finally, GROUP BY CUBE computes a grouping set for each combination of grouping expressions. For example,
+`GROUP BY CUBE (country, city)` is equivalent to `GROUP BY GROUPING SETS ( (country, city), (country), (city), () )`.
+Grouping columns that do not apply to a particular row will contain `NULL`. For example, when computing
+`GROUP BY GROUPING SETS ( (country, city), () )`, the grand total row corresponding to `()` will have `NULL` for the
+"country" and "city" columns.
+
+When using GROUP BY GROUPING SETS, GROUP BY ROLLUP, or GROUP BY CUBE, be aware that results may not be generated in the
 
 Review comment:
   I think we can keep to the prevailing convention of plain text for inline keywords, with all caps providing the formatting distinction. 
   
   The underlying point is taken though—a series of keywords like this makes the text a little hard to scan.  One possible workaround, if the problem warrants it, would be to restructure the sentence to put the keywords in bullet format. For example --
   
   > When using the following, be aware that results may not be generated in the order that you specify your grouping sets in the query:
   > 
   > - GROUP BY GROUPING SETS 
   > - GROUP BY ROLLUP
   > - GROUP BY CUBE
   > 
   > If you need results... "
   > 

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